1. Field of the Invention
This invention relates to color proofing, and more particularly to the production of color proofs that include independent simulation of the expected image dot gain of a color printing press.
2. Description of the Prior Art
Modern color printing of high-definition accurate-color pictures is accomplished by a four-step printing process comprising sequential application of standardized ink colors to a print medium in a uniform pattern location but with varying pattern size. FIG. 1(a) shows a typical grid containing colored dots of equal size thereon. On an actual printed picture the dots are present but the grid lines are not; the grid lines are shown only for the purpose of explanation. A multiplicity of equal-sized dot patterns as shown in FIG. 1(a) printed on a white page would produce a grey-looking region as viewed by the human eye. Therein, Y means yellow, B means black, M means magenta, and C means cyan. It is to be understood that the dots are very small in size, so as to be indistinguishable to the unaided human eye. Large regions of such sets of dots are integrated by the human eye into a continuous-looking image of some shade of grey depending on what percentage of the grid the equal-sized dots fill.
In order to obtain varying hues, as opposed to just varying shades of grey, the different-colored dots are varied in size with respect to each other as shown in FIG. 1(b). The grid section shown would produce a tiny region with a greenish look to the human eye. By combining many thousands of grid sections of varying dot sizes, a multi-colored print is produced in accordance with a desired pattern (i.e., a colored picture). The dot sizes and their relative magnitudes must be controlled very accurately, since a mere 2% variation in relative dot size can produce a humanly discernible change in color.
In standard practice, the colored picture printing process starts with the separation of a colored photographic image into four separate image maps containing appropriate variable weightings (i.e. dot sizes) of yellow, black, magenta, and cyan respectively.
Samples of small sectors of such maps are shown in FIGS. 2 (a-d); again, the grid lines are shown only for the purpose of explanation--they do not actually appear on the maps. These separate-colored image maps are referred to as "separations", the preparation of which has been standardized in the half-tone printing industry. Once the form separations have been made for a particular picture, they are used to generate four corresponding master printing plates by well-known optical exposure techniques. By observing close mechanical tolerances, the printing plates are then inked and sequentially applied to the same carefully-aligned print paper. The result is a high-quality multicolored print corresponding closely to the original photograph.
There are a variety of variables that must be controlled accurately in order to produce high-quality prints using the approach described above, e.g.: (1) "dot range" (i.e. the relative extremes of dot size); (2) the "solid density" (i.e. the color density of a solid patch); and (3) "dot gain" (i.e. the small increase in dot size from the original separations to the final print due to normal ink spreading during printing).
Before a printing run is done, it is usually desirable to produce a "proof" print from the four separations using photographic techniques. Since the object of a proof is to predict closely what the final prints will look like, it is necessary to simulate the dot gain phenomenon during the proof making process. The currently-used technique for simulating dot gain comprises exposing the proof photographic paper to the combined separation patterns for a longer-than-necessary time, thus causing the respective re-imaged dots to "bleed" outwardly to diameters somewhat larger than those of the original separation. Unfortunately, exposure time has a markedly nonlinear effect on photographic receptors with regard to various factors such as image development process grain size, color density (i.e., color shade), and even to some degree color hue. Thus, increasing exposure time to obtain the desired dot gain similation can adversely affect other factors that impact the clarity and fidelity of the resulting proof photograph. Some compromise of proof quality must therefore be accepted using currently-known techniques.
It is therefore an object of the instant invention to provide a method and apparatus for simulating dot gain during the process of making photographic proofs without unnecessarily altering the photographic exposure time of the proof film.
In the related art, U.S. Pat. No. 4,708,459, issued to Cowan et al. on Nov. 24, 1987, discloses at Column 12, line 29 through column 15, line 15, known techniques for accomplishing some control over dot gain, including exposure control, working voltage control, and use of a transparent spacer, but the independent dot gain control technique of the instant invention is not shown or suggested.
U.S. Pat. No. 4,229,097 issued to H. N. Vulmiere et al. on Oct. 21, 1980, describes a system for creating enlarged half-tone or continuous-tone negatives for four-color printing. This system, as summarized at column 2, line 26 through column 3, line 26, utilizes various individual lenses, a condensor, an auxiliary light source, frosted discs, and other apparatus in combination. Nowhere therein is there disclosed or suggested the apparatus or method of the instant invention.
U.S Pat. No. 4,239,385, issued to Hujer on Dec. 16, 1980, discloses, at column 3, line 28 through column 4, line 56, an optical system for controlling exposure employing a liquid crystal element array. The instant invention is not shown or suggested therein.
U.S. Pat. No. 4,025,189 issued to Pugsley on May 24, 1977, discloses apparatus to control the intensity of light used for reproducing half-tone images. This system, as detailed at column 5, lines 1 through 60, employs a PLZT modulator placed between crossed polarizers for the purpose of controlling light transmission. The instant invention is not disclosed or suggested therein.